1. Field of the Invention
The present invention relates to a magnetic disc apparatus for use in controlling the operation of a magnetic head with respect to a target data track or the like.
2. Description of the Prior Art
Relative to servo systems for controlling the position of a magnetic head with respect to a target data track, there have been proposed a variety of methods heretofore, including a sector servo system and a servosurface servo system.
In the sector system, as disclosed in FIG. 2 of U.S. Pat. No. 3,812,533, a multiplicity of data tracks are arrayed on a magnetic disc, and servo tracks are so formed that one is existent in a circumferential portion of each data track.
Meanwhile in the servo-surface servo system, as disclosed in FIG. 1 of U.S. Pat. No. 4,072,990, servo tracks are formed on one entire surface of one of the magnetic discs superposed with a predetermined space.
It is also known that, as described in U.S. Pat. No. 3,737,883 and so forth, an external position sensor such as an optical sensor is employed to perform a seek operation before reading out a servo signal written on a disc.
In the sector servo system where control is executed for each of the sectors individually, a rotation waiting time is needed until a desired sector arrives and a servo signal is read out therefrom, so that the required displacement time of the head is rendered long.
Furthermore, since a servo track is formed in a circumferential portion of each data track, it is impossible to write the data in the entire circumferential length of the data track, and a margin is required for embedding a servo pattern.
Meanwhile in the servo-surface servo system where one whole surface of a magnetic disc is used for servo tracks, the utilization efficiency of the magnetic disc is low with regard to the data tracks.